Hot-dip galvanized steel sheets have been used as steel sheets for home electric appliances, steel sheets for building materials, and steel sheets for automobiles. However, electrogalvanized steel sheets are mainly used for applications such as casings of home electric appliances, in which a surface appearance is regarded as being important, but hot-dip galvanized steel sheets are little used because unlike electrogalvanized steel sheets, a uniform surface appearance cannot be obtained due to spangle patterns or dross adhesion. However, hot-dip galvanized steel sheets can be produced through an annealing step and a coating step in one line after cold rolling and thus can be produced at lower cost than electrogalvanized steel sheets produced through an annealing step and an electrogalvanization step in different lines after cold rolling. Therefore, if the above-mentioned problem can be resolved, hot-dip galvanized steel sheets are lower-cost materials than electrogalvanized steel sheets for applications such as castings for home electric appliances.
On the other hand, there have recently been developed techniques for improving a surface appearance by imparting predetermined roughness to a galvanized surface of a hot-dip galvanized steel sheet. For example, Patent Literature 1 discloses a technique for achieving a uniform surface appearance required for automobile applications, particular for decreasing “a feel of roughness”, by controlling centerline mean roughness Ra and mean peak spacing Sm of the surface of a hot-dip galvanized coating film in predetermined ranges. In addition, Patent Literature 2 discloses a technique for improving sharpness after painting by adjusting roughness parameters, such as Wca (height of filtered centerline waviness) and PPI (number of peaks of projections within a predetermined range) of a coated surface, within predetermined ranges.
Patent Literatures 1 and 2 do not describe conductivity and corrosion resistance of hot-dip galvanized steel sheets. With respect to corrosion resistance, there has been a technique of securing white rust resistance by coating a galvanized surface with a chemical conversion coating film, and corrosion resistance has been achieved by chromate treatment with a treatment solution containing chromic acid, dichromic acid, or a salt thereof as a main component.
Chromate treatment uses hexavalent chromium which is a pollution control substance, but hexavalent chromium is treated in a closed system in a treatment process and completely reduced and recovered without being released to the natural world. In addition, chromium elusion from a chromate coating film can be substantially completely eliminated by the action of sealing with an organic coating film, thereby causing substantially no pollution of environments and human bodies with hexavalent chromium. However, from the viewpoint of recent global environmental problems, use of hexavalent chromium tends to voluntarily decline, and products tend not to contain hexavalent chromium as much as possible.
Therefore, in order to prevent the occurrence of white rust on galvanized steel sheets, many treatment techniques without chromate treatment, i.e., chromate-free techniques, have been proposed. An example of the techniques is to form a surface treatment coating film on a galvanized steel sheet using an inorganic compound, an organic compound, an organic polymer material, or a composition containing a combination thereof.
However, it is difficult for conventional chrome-free techniques to satisfy both the conductivity and corrosion resistance of a surface-treated hot-dip galvanized steel sheet having a surface treatment coating film further formed on a galvanized steel sheet. A typical chromate-free surface treatment agent imparts corrosion resistance by forming a dense reaction layer with zinc of the surface of a galvanized steel sheet. Therefore, when a surface treatment agent having high reactivity to a galvanized surface is used for chromate-free treatment of a hot-dip galvanized steel sheet which is provided with proper irregularities in consideration of surface appearance, a thick coating film is formed on the galvanized surface, and thus excellent corrosion resistance is exhibited, but satisfactory conductivity cannot be obtained. On the other hand, when a surface treatment agent having low reactivity to a coating is used, particularly, a thin coating film is formed on a projecting portion, and excellent conductivity is achieved, but satisfactory corrosion resistance cannot be obtained.
As described above, a hot-dip galvanized steel sheet satisfying all of such a uniform surface appearance that it can be applied to casings of home electric alliances, and excellent conductivity and corrosion resistance has not been produced. The present invention is aimed at resolving this problem.
In addition, with respect to conventional chrome-free surface treatment agents, problems described below, which have been unrecognized, are being newly recognized.
A first problem is to improve corrosion resistance at a processed portion of a surface-treated galvanized steel sheet. A surface-treated galvanized steel sheet is processed (cutting, bending, or part welding) to form a product. In bending a surface-treated galvanized steel sheet, a coating on the surface side of a bend portion is extended. In this case, the surface treatment coating film is extended with the bending and is thus damaged to expose the galvanized surface, thereby causing the problem of deterioration in corrosion resistance at the exposed portion. In particular, in the case of bending, damage to the film and coating continuously occurs, not locally occurs as in extrusion, and thus it is very difficult to achieve corrosion resistance at the processed portion.
A second problem is to secure solvent resistance of a surface treatment coating film. In the processing step, oil stain adhering to the coating film surface or a symbol written with a magic marker may be wiped off with a solvent. In this case, the phenomenon of peeling or whitish discoloration (whitening) of the surface treatment coating film with the solvent is often observed. Peeling of the surface treatment coating film makes it impossible to achieve corrosion resistance of the (surface-treated) galvanized steel sheet, while whitening of the surface treatment coating film degrades the appearance quality.
A third problem is to secure paintability of the surface treatment coating film. The surface-treated galvanized steel sheet processed as described above may be surface-washed with an alkaline cleaner (alkaline degreasing) and then painted. Therefore, upper coating paintability after alkaline degreasing may be required. However, under the present situation, known documents of investigation of this property are not found.
A fourth problem is to satisfy both the corrosion resistance of the surface treatment coating film and the storage stability of the surface treatment agent. Recent chromate-free techniques most frequently use a so-called application type in which the surface treatment coating film is formed by application to the galvanized steel sheet and then drying. In order to impart a barrier effect to the thus-formed surface treatment coating film, the surface treatment coating film is required to have predetermined water resistance. This is because the surface treatment coating film which is easily re-dissolved in water has difficulty in securing corrosion resistance (barrier effect). On the other hand, it is industrially important that the surface treatment agent prepared as one liquid containing raw materials at a predetermined ratio can be stably stored. The surface treatment agent is desired to be stably stored without deterioration over a long period of time even when stored at 35 to 40° C. on the assumption of summer temperature. The storage stability requires the surface treatment agent to be prevented from thickening, gelling, or precipitating, i.e., the surface treatment agent is required to have predetermined water solubility, and further the quality at the time of preparation is required to be maintained over a long period of time even after storage.
Some examples of a conventional chrome-free technique are given below. Patent Literature 3 discloses a method in which an aqueous solution containing water-dispersible silica, an alkyd resin, and a trialkoxysilane compound is applied to a metal surface and then dried to form a coating film. In addition, Patent Literatures 4 and 5 disclose a surface treatment method aimed at imparting corrosion resistance to a metallic material by using a water-soluble resin composed of a hydroxypyrone compound derivative, and a method for imparting corrosion resistance to a metallic material using an aqueous solution of a hydroxystyrene compound or a water-dispersible polymer. Further, Patent Literature 6 discloses a technique using a surface treatment agent containing an aqueous resin, colloidal silica, and ammonium vanadate at a specified ratio. However, any one of these techniques cannot reach the development of a coating film which can be replaced for a chromate coating film in order to impart corrosion resistance.
Patent Literature 7 discloses a technique for a surface treatment coating film containing an organic resin and a thiocarbonyl group-containing compound, but corrosion resistance after alkaline degreasing is unsatisfactory. Patent Literature 8 discloses a technique for treating a surface of a metal plate with a treatment solution containing an organic resin, a silane coupling agent, and a solid lubricant in an aqueous lithium silicate solution, but corrosion resistance at a portion processed by bending becomes unsatisfactory because an inorganic component easily forms a hard polymer. Also, since an alkali metal is contained, secondary adhesion of a coating is degraded. Patent Literature 9 discloses a technique of forming a resin coating using an aqueous resin solution containing a carboxyl group-containing polyurethane resin, an ethylene-unsaturated carboxylic acid copolymer water dispersion solution, silica particles, and a silane coupling agent at a specified ratio, but solvent resistance and corrosion resistance at a processed portion are unsatisfactory. Patent Literature 10 discloses a steel sheet having a coating film which contains a urethane resin, a lubricant, an inorganic colloid compound, and a silane coupling agent at a specified ratio, but the steel sheet is designed on the assumption of electrodeposition coating and thus has excellent electrodeposition coating properties while satisfactory corrosion resistance at a processed portion is not achieved.
Patent Literature 11 discloses a surface treatment solution prepared by mixing a silane coupling agent with a urethane resin and adjusting the resultant mixture to pH 2.5 to 4.5, but corrosion resistance after alkaline degreasing is degraded, and solvent resistance is also unsatisfactory. Patent Literature 12 discloses a technique of forming a coating film using a treatment solution which contains an aqueous disperse resin, silica particles, and an organic titanate at a specified ratio, but corrosion resistance at a processed portion is unsatisfactory. Patent Literatures 13 and 14 disclose a technique of forming a coating film by using a treatment solution which contains a specified aqueous epoxy resin dispersion, a urethane resin dispersion, a silane coupling agent, phosphoric acid and/or a phosphate compound, and a compound having 1 to 5 fluorine atoms in its molecule, but there is room for improvement in corrosion resistance after alkaline degreasing and paintability because of a slight shortage of alkali resistance.
Patent Literature 15 discloses a technique of forming a coating film using a treatment solution which contains a specified resin compound, a vanadium compound, and a metal compound containing a specified metal, but corrosion resistance after alkaline degreasing cannot be achieved because of insufficient alkali resistance, and the problem of easy yellowing during heating is not resolved. Patent Literature 16 discloses a technique of forming a coating film using a treatment agent which contains a specified resin compound, a cation urethane resin containing a cationic functional group, a silane coupling agent containing a reactive functional group, a Ti compound, and an acid compound at a specified ratio. This technique can form a coating film having excellent corrosion resistance and fingerprint resistance, but corrosion resistance after alkaline degreasing, corrosion resistance at a processed portion, and solvent resistance are not investigated, leaving room for improvement in these properties. Patent Literature 17 discloses a technique for a surface treatment agent containing at least one aqueous resin selected from cationic and nonionic urethane resins, a specified resin compound, a metal compound containing a specified metal, and water, but alkali resistance, corrosion resistance at a processed portion, and solvent resistance are not investigated and are not sufficiently improved. Patent Literature 18 discloses a technique using a surface treatment agent which contains, at a specified ratio, a cationic urethane, a cationic phenol condensate, and a compound containing titanium and a specified metal, but solvent resistance and paintability are not investigated and are not sufficiently improved.
Further, any of the surface treatment agents of the conventional techniques containing a silane coupling agent tends to have low storage stability. Therefore, even if the surface treatment agents have no problem with appearance after storage, desired characteristics cannot be obtained in many cases, and the above-mentioned problems are not investigated in the conventional techniques.